1. Field of the Invention
The present invention relates to a lens meter for measuring optical characteristics of a lens.
2. Description of Related Art
Conventionally, there is known a lens meter which projects a measurement light bundle onto a subject lens to be measured which is placed on a measurement optical axis, and photo-receives the measurement light bundle having passed through the subject lens by the use of a photo-sensor, and then obtains optical characteristics such as refractive power of the subject lens based on the photo-receiving results. For this kind of lens meter, proposed initially was one which measures optical characteristics in one measurement position (measurement point) of the subject lens, and after that, proposed recently has been one which measures optical characteristics in a plurality of measurement positions (measurement points) at a time within a measurement region covering a wide range extending over a distance portion, a progressive portion and a near portion of the subject lens (see U.S. Pat. No. 5,910,836 corresponding to Japanese Patent Application Unexamined Publication No. Hei10-104120, and other references).
Though the lens meter which measures the optical characteristics in one measurement position is enough for measurement of a single vision lens, it takes a lot of trouble in measurement of a progressive power lens before finding the distance portion and the near portion. Further, it takes much time to obtain refractive power distribution of the progressive power lens.
On the other hand, the lens meter which measures the optical characteristics in the plurality of measurement positions can easily obtain the refractive power distribution of the progressive power lens; however, requirements are extensively increased in order to meet necessity and size of optical members such as a screen and an image forming lens, size of a photo-receiving surface of the photo-sensor, performance of a calculation part, and the like for obtaining refractive powers in the respective measurement positions within the measurement region covering the wide range extending over the distance portion, the progressive portion and the near portion, and thereby the cost of production is raised. Further, since the measurement cannot be performed while bringing heights to a rear surface of the lens within the measurement region to be almost constant, measurement results on the distance portion and the near portion positioned in a peripheral part within the measurement region possesses low reliability.